With the exception of the off-peak return, which, where regulated, must, as a minimum, be available during times defined in the franchise agreement, there are no restrictions in the defining of the peak hours in which peak tickets are required for travel.
The profile of passenger fares that constitutes the peak varies between franchise areas.
Off-peak day fares (old cheap day returns) are unregulated and train operators have the discretion to set the hours of their validity.